1. Field of the Invention
The invention concerns television cameras having image pick-up electron tubes.
An image pick-up tube is a vacuum tube with a frontal surface constituted by a plane photosensitive target on which, by means of lenses or other optical means, there is focused an image that is to be converted into an electrical signal called a video signal.
The tube includes an electron gun placed in the rear of the photosensitive target to produce a narrow electron beam directed towards this target, focusing means to focus this beam on the target, and deflection means to subject the beam (and consequently its point of impact on the target) to a scanning of the surface of the target or of a part of this surface.
To give a video signal representing the illumination of each point of the target, the front face of the target is coated with a transparent electrode that is connected to an output connection terminal at which will be extracted the signal current which, after processing, becomes the video signal. This electrode is called a signal electrode.
This transparent electrode is a thin conductive layer deposited on the signal input plate (hereinafter called an input plate) of the tube, the input plate being made of transparent glass (optical glass). The invention can be applied to the case where the photosensitive target proper is a layer of photosensitive material deposited on the input plate on top of the transparent electrode. It can be applied notably to the case of a target made of photoconductive material.
In the case of a target made of photoconductive material, the signal electrode is biased at a positive voltage with respect to the cathode of the electron gun. Positive charges are created by photoconductive effect when a light image is focused on the target. The quantity of charges at a given point of the target is proportional to the integral of the illumination received at this point. These localized charges migrate towards the surface of the target on the electron gun side under the effect of the internal electrical field in the photoconductive material. The charges localized at a point of the target are compensated for by the arrival of electrons when the beam from the electron gun touches this point. This then prompts the flow, through the signal electrode, of a corresponding quantity of negative charges, by capacitive discharge, through the material of the target. The time-related variation of flow of charges forms a signal current which, as and when the target is scanned by the beam, represents the luminous information received at each point of the target.
2. Description of the Prior Art
One of the technical problems that has to be resolved for making the image input plate of the image pick-up tube is that of the output of the video signal through the signal electrode and then through the external connection terminal to which this signal electrode is connected. For, the transparent signal electrode is deposited on the plate on the side internal to the tube (which is a vacuum tube) while the signal has to be transmitted outwards.
FIG. 1 shows a known structure of a prior art input plate. In this structure, the plate is formed by two transparent glass sheets bonded to each other, the larger one (10) of these two sheets being, for example, circular and having the diameter of the image pick-up tube for which it is the input element, the smaller one (12) of these two sheets having, for example, a rectangular format corresponding to the format of the image which actually has to be observed. This second sheet has, for example, the function of improving the image contrast by anti-halo effect, of adjusting the optical path by the thickness of the sheet or any other function. The input plate is formed in two steps: the first sheet (10), which is the larger one, is first of all polished on both its faces so as to be quite plane. Then it is pierced with a hole for the passage of a connection terminal 14 going through it. The hole is formed outside the useful image field, namely outside the zone which will be covered by the second sheet.
The transparent signal electrode 16, made of indium-tin oxide (ITO) for example, is deposited by sputtering on the internal face of the input plate (the side internal to the tube). Then, the connection pin 14 is introduced into the hole. A solder (18) is then made between the pin and the deposited electrode. This is a vacuum-sealed solder with an alloy that has a low melting point and is preferably indium-based. As shown in the figure, the solder covers the pin and a part of the transparent electrode that surrounds it.
The photoconductive layer 20 is then deposited. There is perfect electrical contact between the photoconductive layer 20 and the transparent electrode 16 and, hence, also with the connection pin 14.
In the case of FIG. 1 (but this is not obligatory) a second sheet 12 of polished glass is bonded with a transparent bonder to the external face of the first sheet. The bonding may take place, for example, only when the first sheet is definitively mounted and fixed to the tube for which it forms the input element.
After the bonding operation, those parts that do not need to let through light are painted black (with a non-reflecting dull black paint). These parts are the lateral edges 24 and 26 of the first and second sheet respectively (the edges perpendicular to the face bearing the photoconductive target and the transparent electrode) and the edges 28 of the first sheet, around the second one, on the external face of the first sheet. On the one hand, this black paint absorbs the light received from the exterior, except on the useful surface defined by the second sheet. Secondly, it absorbs the light reflections inside the sheets. It thus serves to reduce what is called stray light.